Among the most important components of fiber optic transmission systems are connectors and attenuators.
The chief function of a fiber optic connector is to assure a low-loss connection between optical fiber ends and/or between optical fibers and other components of a fiber optic system, such as optical sources (lasers and LEDs), optical receivers (PIN or APD diodes) etc. There are many versions of fiber optic connectors, known as ST(copyright)1, SC, FC, biconic etc.
1ST is a registered trademark of ATandT 
The chief function of an attenuator is to regulate the strength of an optical signal in the transmission system. It is desirable to maintain the strength of an optical signal within a certain range to avoid receiver saturation, or to compensate for variable distances of various receivers from the source or to compensate for aging or other changes in the system.
There are several kinds of attenuators on the market. They can provide either variable or fixed attenuation over a wide range of signal strength.
Another important aspect of fiber optic transmission systems is the control of optical back-reflections. As an optical signal propagates through the fiber, it passes through a series of interfaces (e.g., between two connectors) which cause a certain portion of the signal to reflect back toward its source. Such back-reflections cause undesirable interference with a signal and thus must be minimized. This is particularly important for high-speed transmission systems and analog signal transmission systems, such as cable TV. There are several methods which are employed to control back-reflections, e.g., optical isolators angle polished connectors, anti-reflection coatings etc.
This invention describes a device called a variable attenuation connector (VAC), which simultaneously addresses all of the above described requirements of a fiber optic transmission system, i.e., a VAC provides connectivity, regulates attenuation and controls back-reflection.
The VAC is a very important development for economic as well as technical reasons. In order to cover all of the functions of a VAC one has to employ a variable attenuator connected by a fiber to the connector. One thus deals with three fiber terminations, i.e., termination at the entry into the attenuator, termination at the exit of the attenuator and the termination of the connector itself. With a VAC only one termination is needed, that of the VAC itself. Elimination of two terminations is a major economic advantage, since terminations cost about $10 each.
A second major savings is in the cost of components. Variable attenuators with characteristics comparable to VAC sell for over $150 in quantities of one hundred or so, while connectors cost about $10-$20 each. The cost of a VAC will be significantly below $100, thus one saves about $100 per attenuator by using a VAC.
The technical advantages of a VAC are also significant. In the so called fiber-in-the-loop networks, which make up the fastest-growing segment of fiber optic markets, it is very important that all receivers in the loop receive similar levels of light signals. This is difficult to achieve since some receivers are closer to sources than others, these will then receive a considerably stronger signal. A VAC will enable cable installers to adjust the strength of the signal for each receiver by simply measuring the incoming signal at the given location and then setting the attenuation at the desirable level.
Also important is the convenience of adjustment at the connector itself. By turning the knurled nut one basically opens and closes the xe2x80x9clight faucetxe2x80x9d at the point of need, just as one regulates flow of water at the end of a pipe.
The VAC will thus find wide usage in fiber-in-the-loop systems.
A VAC is essentially a modified optical connector which permits linear motion of a fiber-containing ferrule to create an air gap between two mating connectors. The attenuation of the signal strength is proportional to the size of the air gap.
The linear motion which creates the gap is accomplished by a fine screw mechanism attached to the ferrule. The size of the gap is regulated by the number of turns of a knurled nut positioned at the rear of the VAC. In this way signal attenuation ranging from 0-40 db can be achieved.
The extent of back-reflection is regulated by the angle polishing of the ferrule face. Once the gap is created, the light is reflected at an angle, which makes it miss the core of the fiber polished at an angle. Thus, only a minute fraction of back-reflected light enters the fiber. Back-reflections below xe2x88x9250 db have been shown.
We have thus invented a device that performs multiple functions in fiber optic transmission systems, i.e., the VAC acts as a connector, a variable attenuator and it reduces back-reflections.